Considerable interest has been shown recently in the use of polyvinyl alcohol water-soluble films for packaging and bags. Such products have a number of uses. Packaging materials contaminated by toxic or harmful materials such as biocides, preservatives, bleach, fertilizers and detergents, must be disposed in an environmentally safe way. Under increasing pressure of environmental regulation, it is often difficult and expensive to dispose of empty packages or bags which contain residues of toxic or harmful materials. It is highly desirable for the safety of the user of such toxic or harmful materials to use water-soluble polymer films as packaging films. Most of these chemicals are in powder or dust form and a user may breathe or be exposed to them. To protect a user from these chemicals, water-soluble bags can be used. When the chemical product is needed, the package is immersed in water and the package dissolves in water with its content. Airborne contamination is therefore eliminated.
For convenience of use and to reduce handling, many chemical products are packaged in unit containers or bags having a pre-determined quantity. For example, products such as dyes, detergents and cement are packaged in unit containers or bags. The use of such containers or bags permits measured amounts of the packaged chemicals to be used without handling the chemicals directly, protecting users from being exposed to the chemicals. Water-soluble films are suitable materials to prepare such unit containers or bags. When the chemical product is needed, the container or bag is immersed in water and the bag and its content dissolves in water.
In the healthcare field, articles such as bedclothes from hospitals, particularly from infectious disease hospitals, have to be sterilized for reuse. It is important to protect workers from contact and exposure to the used articles. Bags prepared from water-soluble films can be used as a container and put into a sterilizer directly without opening, to eliminate contact with infectious disease bacteria.
Water-soluble bags prepared from polymeric films are particularly useful where the contents are toxic or should for other reasons not be touched.
Water-soluble films are also widely used in immersion printing processes. These are processes for decorating products with complex curving surfaces. Water-soluble films can suitably be used as carriers of immersion printing plates. In immersion printing processes, controlled water-solubility rate and stiffness are both very important to the films floated on the water surface.
Many techniques have been investigated to modify polyvinyl alcohol to increase the water-solubility rate of films prepared therefrom. They can be summarized as following: (1) degradation of the polymer; (2) design of film structure; (3) new copolymers of polyvinyl alcohol; and (4) modifications of polyvinyl alcohol.
U.S. Pat. No. 6,071,618 (Cook, Jr. et al.) discloses a process for increasing the solubility rate of a water soluble polyvinyl alcohol film. A polymeric film comprises at least one irradiated water soluble layer. In the irradiation process, the film is subjected to an energetic radiation treatment, such as corona discharge, plasma, flame, ultraviolet, X-ray, gamma ray, beta ray, and high energy electron treatment. The molecular weight of the polymer is decreased by the degradation of the polymer through the irradiation process to increase the solubility rate of the polymer. The molecular weight of the polymer is increased by crosslinking of the polymer through the irradiation process to decrease the solubility rate of the polymer. The irradiation equipment and process are very expensive. The patent indicates that the water-solubility rate of the films at 0° C. can be increased.
U.S. Pat. No. 3,387,405 (Iwasyk et al.) discloses a foam structure film of polyvinyl alcohol. A polyvinyl alcohol solution containing gelling agents is mixed with bubbles of air under pressure. The foam is dried to a cellular film to improve the water solubility rate. It is a slow process and difficult to control the gelling conditions.
U.S. Pat. No. 3,157,611 (Lindemann) discloses a polyvinyl alcohol film plasticized by phosphate ester to improve the cold water solubility rate. It also indicates that many plasticizers such as glycerin, ethylene glycol, ethanol acetamide and ethanol formamide have been suggested for polyvinyl alcohol in order to produce a material readily dissolved in water but that they are not effective in producing polymeric materials with improved cold water solubility. The plasticizers disclosed in this patent have the following disadvantages: they are humectants which absorb moisture in high humidity conditions to make them become tacky and weak; or they are volatile so that they are easy to migrate to their surface. All these disadvantages will affect the water solubility rate and other physical properties of the film. The fact that phosphate esters are not highly compatible with polyvinyl alcohol results in the former tending to migrate to the surface of the latter.
U.S. Pat. No. 2,948,697 (Robertson) discloses that polyvinyl alcohol films plasticized by combination of polypropylene glycol and phosphate esters are improved in their water solubility rates both at high and low temperatures. But they still suffer two problems, namely poor compatibility between polyvinyl alcohol and plasticizers, and the migration of plasticizers to the surface of the films.
U.S. Pat. No. 3,106,543 (Milne) discloses a polyvinyl alcohol modification method: an ethylene oxide is reacted with aqueous polyvinyl alcohol solution to obtain a hydroxyethylated polyvinyl alcohol. The films prepared from the modified polyvinyl alcohol do not contain any plasticizers. Because the process must be carried out in extremely dilute aqueous polyvinyl alcohol solution in an autoclave, it is expensive.
U.S. Pat. No. 3,505,303 (Lindemann) discloses a film made of modified polyvinyl alcohols, which is clear, colorless and readily soluble in cold water. The patent indicates that the solubility of a film can be raised by modifying polyvinyl alcohol through copolymerization, but the solubility of a film in cold water is not increased. The films disclosed have high solubility in cold water. Under alkaline condition, acryl amide is added to some of the hydroxyl groups of the polyvinyl alcohol molecules by an alkene addition reaction. Because the concentration of the polyvinyl alcohol solution is 10-20%, the process is expensive. In addition, the acryl amides must be recovered.
It would be desirable to provide water-soluble films' having the characteristics of controllable solubility rate and rapid cold water solubility, even at 0° C.